Date

2014

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Chemical Engineering

First Adviser

Chaudhury, Manoj K.

Other advisers/committee members

Klein, Andrew; Silebi, Cesar A.; Berger, Bryan W.; Ferguson, Gregory S.

Abstract

Surface science is widely applied in different engineering/scientific fields by exploiting the various physical and chemical properties of interfaces. Here, we explore the electrical properties at hydrophobe/liquid interfaces and the mechanical properties of hydrogels.Most solid surfaces become charged when placed in contact with liquids. This interfacial charge is critical in practical applications such as colloidal suspensions and microfluidic devices. In order to study the charge at the hydrophobe/liquid interface, we developed a simple method to determine the zeta potential (an indication of surface charge strength) of planar hydrophobic surfaces by combining electroosmosis and capillarity. We showed that the measurement of the centerline velocity of the liquid inside the channel is enough to deduce the zeta potential of the surface. This method was further utilized to investigate the basic physics of the charge origin at the hydrophobe/liquid interface. Negative zeta potentials were observed on apparently passive nonpolar hydrocarbon and fluorocarbon surfaces when they are in contact with polar liquids (water, ethylene glycol, formamide, and dimethyl sulfoxide). The current models of charging via the adsorption of hydroxide ions on the interface or the dissociation of pre-existing moieties are not sufficient to illustrate the experimental observations. We hope that these results will inspire further experimental and theoretical studies in this important area of research that has potential practical implications.On the other hand, mechanical properties of surfaces are also important from an adhesion perspective. A side project focuses on investigating the adhesion between thin hydrogel films and flat-end rigid studs. We designed a composite material that was composed of a polyvinyl alcohol (PVA) hydrogel coating covalently bonded to a thin polydimethylsiloxane (PDMS) film. This PVA coating passed a stability test and was characterized by high resolution x-ray photoelectron spectroscopy (XPS) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). This hydrogel layer was found to lower the removal shear stress of a silanized glass cube by ~60% (compared to a thin unmodified PDMS film). This reduction of the adhesive shear stress was presumably due to the lubrication of water kept in the swollen PVA gel.

Share

COinS